The search for the molecule or molecules responsible for the formation, proliferation, differentiation and maintenance of tissue and organs, such as neurons and related neuronal cells and tissues, has been extensive as there is a tremendous need for factors useful for treating conditions involving degradation or damage to these tissues. A Xenopus protein previously identified in embryos, cerberus, appears to be involved in induction of the head. Bouwmeester et al., Nature, 382:595-601 (1996).